Thermionic cathode structure



May 21, 1940.

T. A. ELDER THERMIONIC CATHODE STRUCTURE Filed March 11, 1959 2Sheets-She'et 1 VIII/177mm Ill/1771i Inventor Tho'm as AB! der; by W miim May 21, 1940. T. A. ELDER THERMIONIC CATHODE STRUCTURE Filed March 11,1939 2 Sheets-Sheet 2 Fig. 5.

Inventor". Thom as A.E|der",

- His Attorney.

Patented May 21, 1940 UNITED STATES ATENT OFFiC 2.201.120 'rnsamomccarnons smucruna New York Application March 11, 1939, Serial No. 261,395

9 Claims.

The present invention relates to electrical discharge devices andcomprises, in particular, an improved cathode structure. As aconsequence of my invention I have provided thermionic cathodes whichhave improved characteristics and, in particular, are so constructedthat more efl'ective utilization will be made of electron emission fromthe entire emitting surface of the cathode. Y

Electronic discharge devices heretofore have been provided withribbon-shaped, electronv emitting members which are arranged to beheated to emitting temperature by passage of current. Such cathodes, byreason of differences of potential between their terminals due to theheating current, have not been characterized by uniformity of electronemission throughout their surface. Concentration of current occurs atthe negative terminal of a cathode when the go cathode is heated bypassage of current. In-

equalities in resistivity leading to inequalities in temperature alsoare a source of non-uniformity. Unequal distribution of electronemission in a thermionic cathode leads to shortened gg-life.Difficulties also were presented in con-v structing and supporting suchcathodes in order torender them resistant to deformation during use.

Separately heated thermionic cathodes having so- 'a cellular or hollowstructure also are characterized by inequalities of electron emission.Electron emission is aptto occur preferentially from the more accessibleparts of such cathodes, that is, near an orifice or orifices.

In accordance with my present invention, I

have provided improved ribbon-shaped filamentary cathodes which areheated by a separate heater instead of bypassage of current, and henceare free from such disadvantages.

(o Another feature of novelty is a construction which is adapted toresist deformation.

Embodiments of my invention will be shown in connection with theaccompanying drawings in which, respectively, Fig; 1 is a sideelevation,

partly in section, of a rectifier provided with such improved cathode;Figs. 2 and 3 are longitudinal sections of two modifications of cathodesin which the emitter is a helical ribbon 50 which is provided with aseparate heater; Fig. 4 is a top view of the cathode shown in Fig. 3;Fig. 5 is aside elevation, partly in section, of a. modified electrodeconstruction including a cathode embodyingmy invention; Fig. 6 is asectional .view taken at the region indicated at,

H; and Figs. 7 and 8 are diagrams of electrical connections.

My invention may be embodied in a cathode comprising a ribbon, whichpreferably is of helical configuration. The ribbon is supported, as willbe described, with adjoining turns spaced apart so as to provide an openstructure. Preferably, the ribbon itself is made of mesh or otherforaminous materials. The cathode preferably is coated with alkaline,earth oxide, or other suitm able material for enhancing electronemission.

In cathodes constructed in accordance with my invention the betterutilization of electron emission from the entire emitting surface of thecathode, due to its open structure, makes possible 15 a' reduction insize of cathode for a given power rating. Its construction also makespossible a reduction of time required for the cathode to assumeoperating temperature when starting from a cold state and an increase oftime required to cool to the cold state. This latter the usual basestructure! which'is provided with contactplugs 3 as well understood. Theenvelope is-provided with acharge of ionizable gas,

ordinarily at reduced pressure. The gas content may be argon, neon,xenon, helium, nitrogen, or other of the class 'of so-callednon-condensible gases, or it may consist of a vapor of a a material suchas mercury, or caseium, which is condensed at ordinary temperatures, orof a mixture of such gases and vapors. A quantity of mercury isindicated at 5 which is a source of mercury vapor. Gas pressures ofabout one to several hundred microns ordinarily are suitable foryielding sufficient ionization to result in an arc-like discharge. Forexample, an otherwise evacuated ,bulb containing a. quantity of mercuryat 40 C. bulb temperature will contain mercury vapor at a pressure offive or six microns. The anode 7, carried by a stem 8, leading to anexternal contact 9, may consist of metal, graphite, or other suitablematerial, and may be of any well known construction. The assembly I,consisting of the cathode and auxiliary parts, is supported by a glassstem II, which also carries the usual getter capsule l2 and exhausttubulation.

As shown in Figs. 2 and 3, the assembly oomprises a helical ribboncathode l4, a heater l5, and a surrounding heat shield I6. The cathode[4, shown in Fig. 2, consists of crimped wire mesh material. Thecrimping increases the surface available for electron emission, butuncrimped structures also may be used as shown in Fig. 3. The cathodel4, which may consist of nickel, molybdenum, tungsten, or other suitablemetal, is coated with electron-emitting materials as, for example, amixture of barium and strontium oxides in accordance with well knownpractice.

The cathode I4 is supported by a frame which may consist of twolongitudinal metal rods l8, IQ of nickel, or other suitable metal, whichare insulated from the shield l6 by the insulators 20, 2|. A ring-shapedplate 22 may be provided as a. steadying support at the bottom of thecathode structure, tension being applied against this plate by theexternal nuts 23, 24. An insulator I3 is interposed between the plate 22and the heat shield, thus insulating the cathode therefrom. The cathodepreferably should be welded to the rods l8, [9 at the regions ofcontact, the rods acting as current collectors for the respectivesections of the cathode.

The heater l5, which may consist of a suitable refractory metal, forexample, tungsten, is connected at its upper end to a metal cross rod 25which is connected to the support rods l8, I9 whereby current may beconveyed, thus making the several rods and helical cathode a unitarystructure. Contact to the opposite end of the heater may be made by aterminal wire 26 which, as will presently be described, leads throughthe intermediary of one of the contacts 3 to an external source ofheating current.

The heat shield I6 is made up of a plurality of spaced layers which mayconsist of metal. Relatively rigid inner and outer walls may beprovided, as shown, between which is supported metal foil in looselyspaced relation, as described in British Patent 383,645 of November 24,1932. A cover 28, which also has a plurality of spaced metal walls, issecured to the cylindrical part of -the shield by a reenforcement 29. Anorifice 30 is provided in the cover for the passage of electron currentemitted by the cathode. Other heat shield structures may be employed forthe cathode as well understood.

The construction of the cathode shown in Figs. 3 and 4 is similar inmost respects to the cathode shown in Fig. 2. The helical cathode l4 inthis modification is not crimped. It is supported by the rods [8 and I9and rests at its lower end directly on the insulators 32, 33. Aninsulator 34 is provided for the heater terminal 26. The cathode l4 alsois coated with suitable material for enhancing electron emission. Theorifice 35 (Fig. 4) is annular. The ribbon l4 also is indicated in thedrawings as consisting of mesh material, but it may consist ofnon-foraminous metal. In both structures shown in Figs. 2 and 3, theribbon is arranged flatwise or crosswise with respect to heat radiationsemitted by the elongated filamentary heater, thereby increasing thedirect heating of the cathode. Heat also is received by radiation fromthe interior of the heat shield which is highly heated.

In the modification illustrated by Fig. 5, a plurality of orifices areindicated at 31, 38, 39 and 40 ports 43, 44 terminating in a clampingring 45 which is mounted on the stem II. The shield 42 may function as agrid, and when so used preferably is provided with an inner ring 48provided with an orifice 41. In a rectifier the ring 46 may be omitted.

In this modification the anode 48 consists of a shallow cup, which mayconsist of graphite, as indicated, or of metal or other suitablematerial. abutting on and nearly closing the upper end of the outershield 42. A plate 49 nearly closes the lower end of the shield 42. Thesurrounding envelope has been omitted in the drawings. In contains a gasor vapor at low pressure, as described in connection with Fig. 1.

Two forms of electrical connections are shown in Figs. 7 and 8.Referring to Fig. 7, the heater I is connected by the conductors 50, 5|to a. source of energy, as represented by the secondary winding of atransformer 52. The cathode supports I8, l9 and the cross rod 25 areincluded in this circuit. The cathode l4 and the anode l arerespectively connected to a load circuit, as represented by theconductors 53, 54, the latter containing a resistor 55 as representativeeither of a load, or of a limiting resistance for a load which is notshown. Fig. 7 shows a high resistance element 56 (e. g. 10,000 ohms ormore) in a circuit 5'! connected between the cathode l4 and the shieldIE, but this circuit and resistance may be omitted, the heat shieldbeing electrically insulated from the cathode. A suitable source ofelectric energy is assumed to be connected to the terminals 58 duringoperation. Two of the con tacts 3 of Fig. 1 ordinarily would be includedin the circuit conductors 50, Si, a third completing the load circuit 53and a fourth connecting the resistance 56 in circuit if it is locatedexternal to the tube.

In the arrangement of Fig. 8, the circuit from the transformer to theheater I5 is completed by the conductors 59, 60, and this circuitincludes the heat shield 16, but not the supports or other part of thecathode l4. As in the case of the system of Fig. 7, a resistor 56 ofhigh ohmic value may be connected between the heat shield and thecathode, or, if desired, this connection may be omitted, the shield thenbeing electrically leftfloating, that is, insulated from the cathode.

Electrical connections also have been indicated in Fig. 3, the secondaryof the transformer 52 being connected between the heater terminal'26 andthe shield terminal 6|, the cathode and the shield being connectedthrough a high resistance 56 (which, as stated, may be omitted) to thecathode. The cathode conductor 53 and the anode conductor 54 terminatein contacts 58 to which an appropriate source of power is connectedduring operation of the device.

What I claim as newand desire to secure by Letters Patent of the UnitedStates is:

l. A structure for use in electric discharge devices comprising aradiation heater, a substantially helical, ribbon-shaped cathode havingturns spaced apart with main heat-intercepting surfaces arrangedcrosswise with respect to the path of heat radiations from said heaterin position to be heated to electron-emitting temperature by saidheater, a load conductor connected to said cathode and a circuitindependent of said load conductor containing a source of current forenergizing said heater.

2. An electric discharge tube structure comprising the combination of anelongated radiation heater, an elongated electron-emitting membersinuously arranged in heat-intercepting relation to said heater, one ormore current-conveying conductors supporting and electrically connectedto said member at a plurality of spaced regions, and means forheat-shielding said member.

3. In an electric discharge tube the combination of an enclosure havingan opening, a' heater located therein but spacially separated therefrom,a helical ribbon having turns spaced apart and surrounding and spacedaway from said heater, a material capable of enhancing electron-emissioncoating the surface of said ribbon, conductive supports electricallyconnected to the several turns of said ribbon, and means forelectrically insulating said supports from said enclosure, and separatecurrent-conveying conductors respectively for said heater and saidribbon.

4. In an electric discharge tube the combination of a heat shield havingan opening, an elongated incandescent filament therein, an oxidecoated,ribbon-shaped cathode of mesh material, means for supporting saidcathode in spaced relation about said heater with the broad surface ofsaid ribbon crosswise the path of heat radiations from said filament andseparate currentconveying conductors for said heater and said ribbon.

5. A discharge tube structure comprising the combination of aheat-insulating enclosure, a radiation heater located therein, anelectron emitter surrounding said heater in heat-intercepting relation,said emitter consisting of a ribbon helically wound with turns spacedapart from one another and being arranged fiatwise with respect to heatradiation from said heater, means electrically insulated from saidenclosure for supporting said ribbon, a material enhancingelectron-emission coating said ribbon and conductors which arerespectively connected to said heater to convey heating current and tosaid emitter to convey load current.

6. A discharge tube structure comprising the combination of an elongatedenclosure having an aperture, a filamentary heater longitudinallysupported at the axis of said enclosure, a ribbon having sinuous turnsspaced apart and arranged in heat-intercepting relation with respect tosaid heater, electronically activating material associated with saidribbon, a plurality of support wires welded to said ribbon at aplurality of regions intermediate the ends of said ribbon, means forinsulating said wires from said enclosure, and electrical conductorsleading respectively to said ribbon and'said heater.

7. An electrical discharge device comprising a sealed enclosure, acharge of gas therein at sumcient pressure to permit of the operation ofan arc-like discharge therein, cooperating electrodes including an anodeand a cathode, said cathode consisting of a ribbon helix, an elongatedradiation heater which is axially positioned within said helix, meansfor heat-insulating said cathode, and means for separately conductingcurrent to said heater and to said cathode.

8. An electrical discharge device comprising the combination of a sealedenvelope, a charge of gas therein at a pressure within the limits ofabout one to several hundred microns of mercury, a ribbon-shapedcathode, a filamentary heater for said cathode, the turns of saidcathode being spaced apart to permit of the passage of a dischargetherebetween and being arranged crosswise with respect to said heater tointercept efliciently heat radiations from said heater, an apertured,electrically insulated heat shield sur-- rounding said cathode, acurrent supply circuit including said cathode for energizing saidheater, and a separate load circuit connected to said anode and saidcathode.

9. In an electrical discharge device, the combination of a filamentaryheater, a ribbon-shaped cathode surrounding said heater and having turnsspaced apart from one another and arranged in heat-intercepting relationto said heater, an apertured, electrically insulated heat shieldsurrounding said cathode, and connected to one terminal of said heater,a load conductor connected to said cathode and independent conductorsincluding said heat shield for conveying heating current to said heater.

THOMAS A. ELDER.

